The United States has confirmed its first case of an aggressive strain of monkeypox, marking a significant public health development.
The California Department of Public Health reported the case on Saturday, involving an individual in San Mateo County who had recently traveled from Eastern Africa.
This announcement was later corroborated by the Centres for Disease Control and Prevention.
The infected individual exhibited symptoms linked to their travel history and is currently recovering in isolation at home, according to a press release from the state health department.
Public health officials are actively tracing those who had close contact with the patient.
This strain, classified as clade I, is recognized for its potential to cause more severe disease than the clade II strain that fueled a global outbreak in 2022.
The CDC noted that the United States is now the sixth non-African country to detect this strain, which previously led the World Health Organisation to declare a global public health emergency in August.
This marked the second time mpox, formerly known as monkeypox, was declared a public health emergency in two years.
Mpox is a viral infection characterized by painful lesions, often accompanied by flu-like symptoms. The disease exists in two distinct clades: the more severe clade I and the milder clade II.
While clade II caused over 32,000 infections and 58 deaths in the U.S. during the 2022 outbreak, clade I has traditionally been confined to Central and East Africa, where it is endemic. However, its recent spread has alarmed health officials.
An outbreak in the Democratic Republic of Congo in 2024 has triggered cases in eight additional African nations.
In August, Sweden became the first country outside Africa to report a case, followed shortly by Thailand. Travel-linked cases have since surfaced in Germany, India, and the United Kingdom, according to the CDC.
Unlike clade II, which was predominantly transmitted through sexual contact in 2022—particularly among men who have sex with men—clade I has primarily spread through contact with infected animals and within households.
According to the CDC, two-thirds of suspected cases between January 2023 and April 2024 occurred in children aged 15 and under.
In Congo, however, a variant of clade I, referred to as clade Ib, has been linked to sexual transmission among female sex workers and men who have sex with men.
Public health experts in the U.S. are monitoring the situation closely. Dr. Marc Siegel, an associate professor at George Washington School of Medicine, stated, “I do expect that we will see cases. I think it will more likely be in the same population that was more at risk in the 2022-2023 outbreak. For the general population, I think this poses very little risk.”
Historically, clade I has been associated with fatality rates as high as 10 per cent in African outbreaks. The ongoing DRC outbreak shows a reduced fatality rate of around five per cent, still significantly higher than the 0.2 per cent fatality rate recorded during the 2022 clade II outbreak. US health officials believe the strain may pose a milder threat domestically due to better nutrition and a more advanced healthcare infrastructure.
Preparedness efforts are already underway in the US. The CDC has issued guidelines for mpox testing and is conducting wastewater surveillance for both clades across the country, including at airports.
The availability of the Jynneos vaccine, which requires two doses for full efficacy, provides additional protection. The vaccine, as well as prior mpox infections, is believed to offer robust defense against severe disease from clade I.
Between May 2022 and January 2023, approximately 1.2 million vaccine doses were administered, yet only 23 per cent of high-risk individuals have been fully vaccinated. Health officials are urging expanded vaccination efforts to mitigate the potential impact of clade I should it begin spreading domestically.
Mpox typically begins with a rash that evolves into fluid-filled blisters before scabbing over. Additional symptoms include fever, swollen lymph nodes, and muscle aches.
In the 2022 outbreak, lesions often appeared in sensitive areas such as the genital and anal regions, as well as inside the mouth, causing severe pain in some cases.
As this more aggressive strain of mpox emerges outside Africa, health authorities are closely monitoring its trajectory and urging vigilance to prevent wider transmission.
